Python Join Same Name function
An operator's execute(), invoke() or modal() methods must return a status as a set(). Also note that the return result must be one of {'RUNNING_MODAL', 'CANCELLED', 'FINISHED', 'PASS_THROUGH', ‘INTERFACE’}

The shortcut to creating a set is to wrap the item in curly braces {}.

def execute(self, context):
    # do stuff
    return {'FINISHED'}